For example, an uninterruptible power supply device (UPS: Uninterruptible Power Supply) is a power supply device that normally charges a storage battery by a commercial AC power supply and upon power outage, converts the power stored in the storage battery to an AC power and outputs the AC power.
Such an uninterruptible power supply device is required to quickly and reliably detect power outage of an AC voltage. Regarding power outage detection, there are various proposals (for example, Patent Literature 1 to 6).